Stem cell factor (SCF) is a growth factor that promotes viability and elicits proliferation and differentiation of hematopoietic progenitor and stem cells. In addition to its effects as a single factor, SCF acts synergistically with other growth factors to stimulate dramatic increases in cellular proliferation. The research objective of this project is to delineate the biochemical mechanisms through which SCF acts as a single factor as well as in combination with other hematopoietic growth factors. The receptor for SCF is the receptor tyrosine kinase (RTK) c-Kit. RTKs initiate increases in gene transcription through multiple pathways including the ras-raf-MAP kinase cascade, the JAK/STAT pathway and the Src-family pathway. Little is known about the role of the JAK/STAT pathway and the Src family pathway in SCF-mediated responses. We therefore examined whether members of the Janus and Src family of PTKs were activated in response to SCF. Our studies have demonstrated that the SCF receptor associates with both JAK2 and Lyn. Further, treatment of cells with SCF induces activation of both kinases. In addition, SCF induces association of the transcription factor Stat1 with c-Kit, as well as increases in Stat1 tyrosine phosphorylation and DNA binding activity. To map sites on c-Kit that associate with JAK2, Stat1 and Lyn, we generated a panel of GST fusion proteins encoding different regions of the c-Kit intracellular domain. These studies have demonstrated that Stat1 associates with phosphorylated tyrosine residues within the kinase 2 region of c-Kit (amino acids 763-925) and that Lyn associates with the phosphorylated juxtamembrane region of c-Kit (amino acids 544-577). Presently studies are directed toward mapping the tyrosine residues mediating interaction of Stat1 and Lyn with c-Kit and determining the role of each in SCF signal transduction through mutational analysis. Lastly, to further define the role of Lyn in SCF-mediated responses, we are presently examining the capacity of progenitor cells from Lyn-deficient animals to respond to SCF in vitro.